A snapshot of the fuels used in the United States for electricity shows that coal-fueled generation provides a little more than 39 percent of all electricity, down from nearly 50 percent in 2006. Filling this gap, natural gas now provides more than a quarter of all electricity, and renewables, including wind and large hydroelectric power, provide about 13 percent. Nuclear power continues to provide around one-fifth of net generation (see Figure 2).

The greenhouse gas emissions associated with different sources of electricity vary significantly, depending on the carbon content of the fuel being used. Carbon dioxide makes up almost 99 percent of the greenhouse gas emissions from electricity generation, and carbon dioxide emissions from coal combustion account for almost 80 percent of total electricity generation-related emissions. The combustion of natural gas and petroleum account for most of the remaining carbon dioxide emissions (see Figure 3). Electricity generation-related greenhouse gas emissions have decreased more than 16 percent since 2007.

Key Generation Technologies in Use

Steam Turbine

Coal-fueled electricity is generated almost exclusively by pulverized coal (PC) power plants. These plants crush coal into a fine powder and then burn it in a boiler to heat water and produce steam. The steam is then used to spin one or more turbines to generate electricity. Natural gas, oil or biomass can be used as a fuel in conjunction with steam turbine technology. Similarly, in a nuclear reactor, fission is used to heat water, which directly or indirectly produces steam to drive a turbine and generate electricity. Large coal and nuclear steam units on the order of 500 – 1000 MW or greater are typically used to provide baseload [1][6] generation, meaning that they supply electricity nearly continuously.

Combustion (or single-cycle) turbines are another widespread power generation technology. In a combustion turbine, compressed air and burning fuel (diesel, natural gas, propane, kerosene, biogas, etc) are ignited in a combustion chamber. The resulting high temperature, high velocity gas flow is directed at turbine blades that spin the turbine and common shaft, which drives the air compressor and the electric power generator. Combustion turbine plants are typically operated to meet peak[2][9] load demand, as they are able to be switched on relatively quickly. Newer turbines are able to be switched on and off frequently, so they can provide a firm backup to intermittent wind and solar on the power grid if needed. The typical size is 100 – 400 MW.

A basic combined cycle power plant combines a single gas (combustion) turbine and a single steam unit all in one, although there are other possible configurations. As combustion turbines became more advanced in the 1950s, they began to operate at ever higher temperatures, which created a significant amount of exhaust heat. In a combined-cycle power plant, this waste heat is captured and used to boil water for a steam turbine generator, thereby creating additional generation capacity. Historically, they have been used as intermediate[3][12] power plants, generally supporting higher electricity use during daytime hours. However, newer natural gas combined cycle plants are now providing baseload support.

The primary end uses of electricity vary by sector. In the residential sector, space heating, water heating, space cooling and lighting together account for more than half of household electricity use (see Figure 8). In the commercial sector, lighting is the largest electricity end use (see Figure 9). In the manufacturing sector, half of all electricity use is for powering electric motors (see Figure 10).

Historical Trends

Since 1949, U.S. electricity generation has grown dramatically, with an average annual growth rate of 4.2 percent (see Figure 11). Since 2000, however, U.S. electricity generation has grown at an average rate of less than 1 percent. During this time, generation from natural gas has increased at an average annual rate of 4.9 percent, and non-hydro renewable generation has increased at an average annual growth rate of 9.2 percent. Coal generation has decreased at an average annual rate of 1.6 percent, and in 2013 fell below the level generated in 1990.

From 1990 to 2013, U.S. electricity generation-related greenhouse gas emissions grew an average of 0.5 percent per year, with a general decrease in annual emissions over the past several years (see Figure 12). During this time, the proportion of electricity generation-related greenhouse gas emissions from coal combustion, which peaked in 1996 at around 85 percent, has fallen to 77 percent in 2013. The share of emissions from natural gas combustion has grown from around 9 percent in 1990 to just over 21 percent in 2013, and the share of emissions from petroleum has fallen from around 5 percent to a little more than 1 percent over the same period.

From 1990 to 2013, CO2 emissions from electricity generation, electricity generation, and real gross domestic product (GDP) grew at annual average rates of 0.5, 1.3, and 2.5 percent, respectively (see Figure 13). This illustrates that the U.S. economy grew less electricity-intensive per value of output while the electricity generation also became less carbon intensive over this period.

Global Context

Globally, CO2 is the most abundant anthropogenic greenhouse gas, accounting for 76 percent of total anthropogenic greenhouse gas emissions in 2008; the CO2 emissions from fossil fuel use alone account for 62 percent of total greenhouse gas emissions.[4][34],[5] [35]Electricity generation is by far the largest single source of CO2 emissions (see Figure 14).

The generation profile of global electricity production is similar to that of the United States, with coal being the largest energy source for electricity production (see Figure 15). Globally, 5.1 percent of electricity is generated by oil, whereas in the United States oil makes up less than 1 percent. Also, hydropower makes up a larger share of global electricity generation, while the United States gets a greater proportion of its electric power from nuclear. The United States contributes more than one-fifth of global carbon dioxide emissions from electricity and heat production; China and the United States are the largest single emitters (see Figure 16).

C2ES Work on Electricity

Over the past 20 years U.S. electricity generation has become less carbon intensive, and the U.S. economy has grown less electricity-intensive. Further mitigating greenhouse gas emissions from electricity generation will require a comprehensive approach, including lower-, low- and zero-carbon electricity generation technologies, incorporating renewable energy, switching to lower-emitting fuels, coal or gas with carbon capture and storage, and nuclear power, as well as energy efficiency and conservation. Several types of policies can be employed to promote these mitigation techniques, including emissions pricing (e.g. carbon tax[46] or cap and trade[47]), electricity portfolio standards[48] (also known as clean energy standards), emission performance standards[49], financial incentives for clean energy deployment and energy efficiency, and research and development to support innovative technologies.

Our work at C2ES covers all types of electricity-related topics, including policy and regulation, low-carbon technology status and outlook, and technology innovation. We track and inform policymaking at the state, federal, and regional levels, collaborate on research for papers and briefs, blog about current energy issues, and educate policymakers and others with up-to-date online resources about important low-carbon technologies.

Tracking policy - We track policy progress at the state, federal, and international level. Our state maps [50]provide information about which states have implemented policies that promote low-carbon electricity technologies and energy efficiency. We also track and analyze policy at the national level, including what is happening in Congress[51] and the Executive Branch[52].

[1] [96]Baseload generation describes electric power plants that typically run all day and night, seven days a week.

[2] [97]Peak generation describes electric power plants that run only during times of the highest demand. For example, high demand can occur in the morning when many consumers are waking up and switching on electric appliances or on hot summer afternoons when many air conditioners are running simultaneously.

[3][98] Intermediate generation describes electric power plants that typically run only during daytime hours to support higher use of electrical appliances, computers, lighting and so on.